1. Field of the Disclosure
Embodiments relate to collimating light emitted by a fiber via an array of lenslets on a curved surface.
2. Description of the Related Art
Some object detection systems (e.g., collimating optical systems such as the Hyperion lidar project) rely upon the emission of a scanning optical probe beam by a scanning device. When this probe beam strikes an object within a threshold distance (e.g., 100 m) from the fiber that emits the scanning optical probe beam away), light is scattered from the object back to a receiver unit at the scanning device that records the presence of the object and calculates the object's x,y,z position, whereby x and y are determined based on an emission direction of the scanning optical probe beam and z via time of flight technology (e.g., based on a propagation speed of the scanning optical probe beam along with a time differential between emission of the scanning optical probe beam and detection of the scattered light detected at the receiver unit).
FIG. 1 illustrates an example of an object detection system 100. Referring to FIG. 1, a scanning device 105 includes a fiber 110 and a receiver unit 115. The fiber 110 emits a scanning optical probe beam 120, which makes contact with a car 125 that is within a threshold distance (e.g., 100 m). When the scanning optical probe beam 120 contacts the car 125, the scanning optical probe beam 120 is scattered in numerous directions as depicted with respect to scattered light beams 130-155. Some of the scattered light beams (e.g., scattered light beam 145) reach and are detect by the receiver unit 115 at the scanning device 105, after which the scanning device 105 can calculate the x,y,z position of the car 125.
Achieving desired scanning optical probe beam properties is challenging for various reasons. For example, a scanning optical probe beam should range over a wide field of view (FOV) with little divergence; i.e. a high degree of collimation. An example FOV requirement for an object detection system is +/−60 degrees with a beam divergence of 0.1 degree over the entire FOV. Furthermore, the lens system producing this collimation may be configured with a small format (˜10×10×10 mm).
FIG. 2 illustrates an arrangement of the fiber 110 within the scanning device 105 in more detail. With respect to FIG. 2, the fiber 110, which is a single or multi-mode optical fiber, is the light source of the scanning device 105. More specifically, the light source is the end of the fiber 110 into which light has been injected. To vary the angular direction of the fiber 110, a piezoelectric cylinder 200 rotates/vibrates the fiber 110 (running through a center of the piezoelectric cylinder), through a wide variety of angles. Exciting the cantilever (i.e., the fiber 100) with resonant frequency via the piezoelectric cylinder 200 will induce oscillations at the tip of the cantilever with an amplitude that is 100-200 times of the base of the cantilever. An example of the fiber 110 (or cantilever) in motion is depicted with respect to 205, whereby the fiber 110 is being vibrated resulting in angular rotation of +/−45 degrees in orthogonal planes, resulting in scanning optical probe beam 210. More specifically, the scanning optical probe beam 210 illustrates a view of the light emitted from the fiber 110 from in front of the fiber 110, or alternatively a few of the light that is beamed onto the target object (e.g., the car 125), which is then scattered. This fiber tip path is approximately spherical for small angular motion, but becomes elliptical for larger angular motions as the fiber 110 begins to bend, which complicates the design of a collimating optical system.
Light refracted through a lens system at large angles (e.g., with respect to an optical axis or lens axis of symmetry) is typically subject to larger aberrations relative to light that is refracted through smaller angles. This means that the degree of collimation is reduced for light propagating at the larger angles. Typically, beam divergence increases 3× to 10× times at the largest field angles (e.g., 30 degrees, 60 degrees, etc.). This tendency is exaggerated for a lidar system where the fiber tip source is not constrained to travel in a plane but rather travels on a spherical or elliptical surface.